1. Field of the Invention
The present invention relates to a rubber composition for tire treads and a pneumatic tire using the aforesaid rubber composition. More particularly, the present invention pertains to a rubber composition for tire treads which maintains low fuel consumption properties and which is greatly improved in gripping properties particularly on wet road surfaces at a low temperature, and to a pneumatic tire using the aforesaid rubber composition in tread rubber.
2. Description of the Related Arts
Carbon black has heretofore been prevailingly employed as a rubber-reinforcing filler. The reason for the foregoing is that carbon black is capable of imparting a rubber composition with high reinforcing properties and excellent wear resistance as compared with other fillers.
On the one hand, with the social demands for energy saving in recent years, an attempt is made to achieve low heat-buildup of a rubber composition for a tire, that is, low rolling- resistance of the tire for the purpose of saving fuel consumption of automobiles. In this case, consideration is given to decrease in the compounding amount of carbon black or to the use of carbon black having large particle sizes. However, it is known that any one of these methods that are taken into consideration can not escape from the deterioration of reinforcing properties, wear resistance and gripping properties on wet road surfaces.
On the other hand, there is known a silica hydride (wet silica) as a filler which reconciles low heat build-up properties and reinforcing properties, wear resistance or gripping properties on wet road surfaces when used in a rubber composition for a tire. Thus a number of applications for patents on the invention relating to the silica hydride have been filed with the Japanese Patent Office, including, for example, Japanese Patent Application Laid-Open Nos. 252431/1991 (Hei-3), 248116/1994 (Hei-6), 70369/1995 (Hei-7) 188466/1995 (Hei-7), 196850/1995 (Hei-7), 225684/1996 (Hei-8), 245838/1996 (Hei-8), 337687/1996 (Hei-8), etc.
However, a rubber composition which is compounded with silica hydride suffers from a disadvantage of inferior kinetic performance on dry road, because said composition has a low storage modulus, as compared with a rubber composition compounded with carbon black having a specific surface area similar to that of the silica hydride.
There are known, as a method for enhancing the aforestated storage modulus, a means in which the filling amount of the silica hydride is increased, a means in which the specific surface area of the silica hydride is increased, and the like. Nevertheless, any of the above-mentioned means involves a problem of deteriorating the low heat build-up property which is the characteristic of the silica hydride.
There is known, as a method for improving the gripping properties on wet road surfaces, a means in which the glass transition temperature of the rubber component is raised, that is, the hysteresis loss (tan .delta.) at 0.degree. C. is increased. However there arises a problem that as the glass transition temperature of the rubber component is raised, the performance of the rubber composition at a low temperature is deteriorated and as a result, the rolling resistance of the tire containing the the rubber composition is unfavorably increased, thereby worsening the low fuel consumption properties.
In order to solve the above-mentioned problems, there have heretofore been disclosed a variety of techniques such as (1) a rubber composition suitable for a tire tread, which has improved gripping properties on wet road surfaces by incorporating a special silica and milling contrivance (European Patent No. 501227), (2) a rubber composition suitable for a tire tread, which has improved wet skid performance without deteriorating its processability or wear resistance, while preserving its low heat-buildup properties {Japanese Patent Application Laid-Open No.149950/1995 (Hei-7)} , (3) a rubber composition suitable for a tire tread, which has improved workability and gripping properties on wet or semi-wet road surfaces in the regions of low temperature and high temperature {Japanese Patent Application Laid-Open No.59893/1996 (Hei-8)}, (4) a rubber composition suitable for a tire tread, which has improved gripping properties on wet or semi-wet road surfaces in the regions of low temperature and high temperature without impairing its wear resistance {Japanese Patent Application Laid-Open No. 59894/1996 (Hei-8)}, and the like.
Nevertheless, the rubber composition (1) involves a problem in workability (processability) , the rubber composition (2) can not be said to have sufficient wear resistance, and the rubber compositions (3) and (4) involve the problem of requiring a large amount of reinforcing filler and the problem of being inferior in low heat-buildup properties.
On the one hand, there is known that aluminum hydroxide as a reinforcing filler for rubber, although being inferior to carbon black and the like with regard to reinforcing effect, imparts a rubber composition with low fuel consumption properties and favorable gripping properties on wet road surfaces.
With the recent steady trends towards high horsepower, high functionalization and prolonged service life of an automobile, a tire therefor is intensely required which is excellent in maneuverability from the initial stage to and after the middle stage of running. With regard to a passenger automobile in particular, the bias tire has been replaced with a radial tire, and such radial tire is required to have highly excellent maneuverability, accompanying the alteration of the rate of aspect ratio from 82 to 70, further from 70 to 65.
As one of the means for solving the aforestated problems, investigations are being made on a variety of compounding ingredients for rubber to be employed in a tire tread. When attention is paid to a vulcanization accelerator, there is disclosed in Japanese Patent Application Laid-Open No. 87138/1983 (Sho-58) that a rubber composition which is compounded with a specific vulcanization accelerator and a specific antioxidant, improves its resistant to heat cure and also the external appearance thereof at the last stage of running of a tire, but nothing is investigated at all therein on the kinetic performance of the tire. In addition, there is disclosed in Japanese Patent Application Laid-Open No. 139542/1981 (Sho-56) that a rubber composition which is compounded with a specific vulcanization accelerator, improves its burning (scorching) at the time of rubber milling, but nothing is described or suggested at all therein on the performance of a tire. When attention is paid to an inorganic filler, there is disclosed in Japanese Patent Application Laid-Open No. 53003/1998 (Hei-10) that a rubber composition which is blended with alumina improves its maneuverability at the initial stage of running of a tire, but can not maintain the improved maneuverability to and after the middle stage thereof.